WICKED: These Dubai police photos show how a printer was turned into a bomb by Yemeni terrorists.EPA photos

A terrorist turncoat tipped off the United States about the America-bound bomb parcels before they were intercepted last week, it was disclosed yesterday.

The development came as US authorities said they now believe counterterrorism officials in mid-September intercepted packages that were supposed to be a “dry run” for the mail-bomb plotters — including a carton of religious books and pamphlets and a computer disk sent from Yemen to an address in Chicago.

Unlike the potentially deadly packages discovered Friday, that box didn’t contain explosives, but was sent by the terrorists “so they could track how long it took and whether there would be any problems for the package getting through the system,” a US official said.

Friday’s mail bombs were intercepted after radical rat Jabr al-Fayfi began spilling al Qaeda secrets two weeks ago, said British and Yemeni officials.

Fayfi, a Saudi, was captured by US forces in Afghanistan after the 9/11 attacks and held for five years at Guantanamo Bay.

After he was released to Saudi Arabia, he managed to join the shadowy al Qaeda in the Arabian Peninsula group based in Yemen, and was put on a Saudi list of 85 most-wanted terrorists.

But in September, he contacted Saudi officials, saying he wanted to come home, and a private jet was sent to retrieve him Oct. 16. Officials said he gave Saudi intelligence details of the terror-parcel plan, which were then passed on to Washington and London.

The Saudi newspaper Al-Watan reported yesterday that the Saudis were even able to give US investigators the tracking numbers of the parcels.

Yemen officials said they suspect Fayfi didn’t change sides last month, but had been a double agent for Saudi Arabia ever since he rejoined al Qaeda.

US officials hinted at the extent of the Saudi assist when they said President Obama called Saudi King Abdullah on Saturday to thank him for his “critical” assistance in foiling the plot.

Both bombs intercepted in the plot were bound for the United States — one via Britain, the other Dubai.

Authorities say that it was only after that plot was uncovered that they realized the previously sent packages were likely intended to test US security.